planets to be one of three categories of planets, along with dwarf planets and classical planets. The term planemo ("planetary-mass object") covers all three populations. Stern's and the IAU's definition of 'planet' depends on hydrostatic equilibrium – on the body's mass being sufficient to render it plastic, so that it relaxes into an ellipsoid under its own gravity. The IAU definition specifies that the mass is great enough to overcome 'rigid-body forces', and it does not address objects that may be in hydrostatic equilibrium due to a subsurface ocean or (in the case of Io) due to magma caused by tidal heating. Many of the larger icy moons could have subsurface oceans. The seven largest moons are more massive than the dwarf planet Pluto, which is known to be in hydrostatic equilibrium. These seven are Earth's Moon, the four Galilean moons of Jupiter (Io, Europa, Ganymede and Callisto), and the largest moons of Saturn (Titan) and of Neptune (Triton). Ganymede and Titan are additionally larger than the planet Mercury, and Callisto is almost as large. All of these moons are ellipsoidal. That said, the two moons larger than Mercury have less than half its mass, and it is mass, along with composition and internal temperature, that determine whether a body is plastic enough to be in hydrostatic equilibrium. Io, Europa, Ganymede, Titan, and Triton are generally believed to be in hydrostatic equilibrium, but Earth's Moon is known not to be in hydrostatic equilibrium, and the situation for Callisto is unclear. Another dozen moons are ellipsoidal as well, indicating that they achieved equilibrium at some point in their histories. However, it has been shown that some of these moons are no longer in equilibrium, due to them becoming increasingly rigid as they cooled over time. Dysnomia's shape is not known, but it appears to be dense enough that it must have collapsed to form a solid body. Neptune's second-largest moon Proteus has occasionally been included by authors discussing or advocating geophysical conceptions of the 'planet'. It is more extensive than Mimas but is quite far from being round. Current equilibrium moons Determining whether a moon is currently in hydrostatic equilibrium requires close observation, and is easier to disprove than to prove. Earth's entirely rocky moon solidified out of equilibrium billions of years ago, but most of the other six moons larger than Pluto, four of which are predominantly icy, are assumed